1. Field of the Invention
The present invention relates to the field of telecommunications. More particularly, the present invention relates to improving performance in switch based telecommunications networks employing virtual connections, such as switched virtual connections (SVCs). The telecommunications network may include virtual tandem switches employing asynchronous transfer mode (ATM) networks.
2. Background Information
In standard call processing, cross-office delay must be below an acceptable level in order to minimize the duration of silence after a telephone call has been dialed. The signaling channel message processing required for standard call processing is well-studied and well-specified for conventional time division multiplexed (TDM) circuit-switched voice networks. ITU-T, “Specifications of Signaling System No. 7 ISDN User Part”, ITU-T Recommendation Q.766, March, 1993; and Bellcore, “LSSGR: Switch Processing Time Generic Requirements, Section 5.6”, GR-1364-CORE, Issue 1, June, 1995, are specifications discussing such processing. These specifications dictate the cross-office delay requirements for processing of Signaling System No. 7 (SS7) messages.
With reference to FIG. 1 of the drawings, standard call processing employs end offices 10 connected via tandem trunks 12, direct trunks 14, or both tandem 12 and direct trunks 14. Each trunk 12, 14 is a digital service level 0 (DS0), operating at 64 kbps, that is transmitted between the switching offices 10 in a time division multiplexed manner. Each end office 10 connects to its neighboring end office 10 and the tandem office 16 using separate trunk groups. In this system, trunk groups are forecasted and pre-provisioned with dedicated bandwidth, which may lead to inefficiency and high operations cost.
A new voice trunking system using asynchronous transfer mode (ATM) technology has been proposed in U.S. patent application Ser. No. 09/287,092, entitled “ATM-Based Distributed Virtual Tandem Switching System,” filed on Apr. 7, 1999, the disclosure of which is expressly incorporated herein by reference in its entirety. In this system, shown in FIG. 2, voice trunks from end office switches 20, 26 are converted to ATM cells by a trunk inter-working function (T-IWF) device 22, 24. The T-IWFs 22, 24 are distributed to each end office 20, 26, and are controlled by a centralized control and signaling inter-working function (CS-IWF) device 28. The CS-IWF 28 performs call control functions as well as conversion between the narrowband Signaling System No. 7 (SS7) protocol and a broadband signaling protocol. The T-IWFs 22, 24, CS-IWF 28, and the ATM network 30 form the ATM-based distributed virtual tandem switching system. According to this voice trunking over ATM (VTOA) architecture, trunks are no longer statistically provisioned DS0 time slots. Instead, the trunks are realized through dynamically established switched virtual connection (SVCs), thus eliminating the need to provision separate trunk groups to different destinations, as done in TDM-based trunking networks.
The actions necessary in each office are clearly defined upon reception of a particular SS7 message when operating within the standard network. For a normal tandem trunk call flow, the originating end office sends an Initial Address Message (IAM) to the tandem switch through an SS7 network. The IAM message includes a routing address of the tandem office, calling telephone number, called telephone number, and Trunk ID. The tandem switch has a mean processing delay budget of 180 ms as specified in “Specifications of Signaling System No. 7 ISDN User part” (360 ms for 95th percentile) to process the IAM message and to reserve a trunk in the trunk group that is pre-established to the terminating end office.
In voice trunking over ATM (VTOA) technology, a standard time division multiplexed (TDM) tandem is replaced by three components: a trunk inter-working function (T-IWF), a control and signaling inter-working function (CS-IWF), and an ATM network. The three component architecture (i.e., T-IWFs, CS-IWF, and ATM network) requires signaling channel message processing different from TDM processing but must maintain at least the performance of standard TDM-based network processing. That is, these three components should share the 180 ms (mean) budget, as they are considered to be a unique entity, i.e., a virtual tandem switching system. Hence, the time for the ATM network to establish a switched virtual connection (SVC), which is VTOA's equivalent to reserving a trunk, is stringent.
In VTOA architecture, the end offices and the virtual tandem (i.e., CS-IWF) communicate through an SS7 network, as seen in FIG. 2, the same way the switching offices communicate in TDM-based trunking networks. However, control/signaling and through-connect establishment (an SVC through the ATM network) functions reside in the CS-IWF, and the ATM network and T-IWF, respectively. Coordinating the different components adds new message exchanges into the processing.
In the VTOA architecture, the CS-IWFs have two options upon receiving an IAM message. The first option is to send a message to either an originating or terminating T-IWF for initiation of an ATM connection and wait for an “ATM SVC Established” message before sending the IAM message to the terminating end office. The second option is to send the IAM message to the terminating end office at the same time it sends a request to either T-IWF for an ATM connection establishment. It is expected that the ATM connection will be ready before the reception of Address Complete Message (ACM), which indicates that ringing is applied to the callee and the through-connect should be established in the tandem. The second option provides more time for the establishment of an SVC through ATM network. However, an SVC may very well go through several ATM switches, which generally have reasonably large figures for call setup latency. Although some exceptions exist, it would be unreasonable to assume the latency is low because the latency numbers of new switches are yet to be tested, and already deployed ATM switches can be assumed to serve years to come. In other words, for either option there exists a need for fast SVC setup through the ATM network to stay within the standardized delay budget limits.
One solution to the latency problem is to construct an overlay PVP (Permanent Virtual Path) network in the ATM backbone. With a PVP network, only end points of virtual paths require call processing and transit nodes are not involved in the establishment of SVCs. Further, the design of virtual path networks has been well studied and thus many proposed optimization algorithms exist. However, the efficient management of virtual path networks is still a challenging task in practice. Although constructing an elastic virtual path, which resizes itself with the changing traffic conditions, is a promising solution, there is currently no standard procedure for automatically changing the capacity of virtual paths. Consequently, a telecommunications carrier would have to commit to a proprietary solution, which has its own disadvantages. Finally, PVP networks suffer from the drawback of requiring manual rerouting in case of a network failure. In contrast, SVCs are rerouted automatically by the Private Network-Network Interface (PNNI) routing protocol without interference from the management system in case of failures in ATM network. For management and operations purposes, this feature makes the SVCs highly appealing.